Abstract

Congenital nephrogenic diabetes insipidus (CNDI) is a rare inherited disorder, mostly caused by antidiuretic hormone receptor type 2 (ADHR2) gene mutations, which are inherited as X-linked traits. Less than 10% of cases are due to mutations in the aquaporin-2 (AQP2) gene, inherited in autosomal recessive or dominant manner. We report the case of two adult sisters, of 30 and 27 years of age, diagnosed in early infancy with X-linked CNDI. The patients’ sex and family history did not fit in well with this diagnosis, so we sequenced the coding regions of the ADHR2 and AQP2 genes. As expected, no mutations were found in the ADHR2 gene, while we found a compound heterozygosis for two different mutations in the AQP2 gene. A missense mutation (c. 439G>A, p.Ala147Thr), an already known cause of CNDI, and a novel missense putative mutation of an adenine to cytosine at position 551 (c.551A>C), resulting in the substitution of asparagine with threonine at amino acid position 184 (p.Asn184Thr). This second mutation changes a fundamental extracellular Asn-Pro-Ala motif (NPA) of the AQP2 protein, inhibiting its function. Its pathogenicity has been confirmed by in silico predictions and is in line with comparable alterations to the intracellular NPA motif of the AQP2 protein.